Living the Scientific Life (Scientist, Interrupted)

tags: Ross's X Snow Goose, Chen hybrid, birds, nature, mystery bird, bird ID quiz

Rick Wright, Managing Director of WINGS Birding Tours Worldwide, writes:

White waterfowl are an open trap for the unwary -- in many parts of the US, at many times of the year, the most abundant such bird is Mallard, many of the cultivars of which are solid white, large, and chubby, leading continually to their being mistaken for something else. Domestic Graylags, too, are often white, and they, too, are often confused with native wildfowl.

No such danger with our quiz bird, which shows a head and bill shape utterly unlike that of a domestic Mallard or a white Graylag; this is a genuine white goose, which in North America narrows the field quickly to the two currently recognized members of the genus Chen.

The round head, short neck, large eye, rather steep forehead, and rather short, dark-colored bill with a bluish patch at the base are the standard marks for Ross's Goose. Once the rarest of North America's breeding geese, the population of this gentle little bird has increased in tandem with that of its larger cousin; and its range has widened, too, such that there is no longer anywhere in the US that Ross's Goose can't be looked for with reasonably expectation in the winter.

But there's a problem, as there so often is. Snow and Ross's Geese are closely related, and mixed pairs are not at all uncommon. It can be extremely difficult to distinguish between a "real" Ross's Goose and a small, small-billed hybrid or intergrade. There are a couple of things about the individual in this photograph that bother me. The head -- from this angle, at least -- looks big, and not quite properly shaped for Ross's Goose (or for Snow Goose, for that matter). The bill may be a bit long. The shape and extent of the dark patch at the base of the bill is odd, in my experience. And the bill appears to show a distinct blackish edge along the distinctly "open" tomia, creating a noticeable "grin patch" smaller than that on a Snow Goose but larger than that on a classic Ross's Goose. If I saw this bird in the field, this well, I'd almost certainly call it a white Chen showing probable characters of hybridization.

Note well that I wouldn't outright call it a hybrid. Maybe some of the geese that look like this are in fact "pure" Ross's Geese, simply expressing some of the genetic potential of that long-ago Chen that was the ancestor to both Ross's and Snow. And that possibility raises a series of philosophical questions that all lead straight back to the most important one of all: what do we mean by a species -- and who cares? For the birder, or at least for me, it's enough to have seen a bird that stretches what we think of as our knowledge in interesting ways. And this one certainly does that.

Review all mystery birds to date.